This invention relates to a power cable and more particularly, to a flexible power cable for use between motor control devices and the motors they control, which minimizes electromagnetic noise and radio frequency interference.
Electromagnetic noise and radio frequency interference (EMI/RFI) can create problems in the control of electronic circuits. More recently, EMI/RFI have been a problem in variable frequency drive applications. Power cables for variable frequency drive devices have caused EMI/RFI crosstalk on adjacent controls and instrumentation cables.
In the past, equipment manufacturers had only to worry about interference caused by normal AC current flowing in a power supply circuit. With the recent advances in transistor and semiconductor thyristor technology, and their application in variable frequency drives, the type of signals utilized to provide power to the motor from their controllers has changed the source of the EMI/RFI that must be protected against.
Conventionally, power, controls and instrumentation cables were placed in segregated cable support systems such as cable trays, conduits, duct banks or direct burial trenches which were separated by minimum distances as required by particular standards in order to minimize the effects of the electromagnetic interference. For fixed cable applications, the power cable could be manufactured with an overall armor consisting of lead, corrugated aluminum, copper or bronze or with an overall sheath consisting of wires and tapes made of copper, aluminum, bronze or steel. This reduces the EMI/RFI transmission by the power cable.
Conventional power cables have been manufactured with standardized levels of insulation thicknesses which were not calculated to handle the additional voltage and current spike levels produced by the new generation of controls. Thus, voltage and current spikes may damage the conventional cables and result in motor controller, cable and motor failures.
On equipment with moving sections such as cranes, machine tools, and robots, the power, control and instrumentation cable types are typically placed in close proximity on mechanical cable handling equipment such as festoons, reels, cable tracks and tenders. On this type of equipment, there is limited amounts of separation, if any, and the cables cannot have a solid armor or taped sheath which are not designed to flex. Equipment manufactures have, in the past, utilized standard unarmored or unshielded four conductor flexible motor feed cables in these types of applications. The use of four conductor power cable configurations limits the ability of the cable manufacturer to take advantage of the optimum cancellation effects of trefoil conductor assembly.
Adding an overall armor or tape sheath in order to minimize the effects of the EMI that was produced by the normal AC currents flowing in the power circuit is generally limited to the fixed applications. The cable with an overall armor or tape sheath cannot be applied to a flexible cable application because the extra armor or sheath layer is not designed to be flexible. An armored cable will not flex and a tape sheath will generally only flex to a limited amount during which the tapes will separate and destroy the sheath and cable.
In the published prior art, DE-A-3151234 discloses a flexible power cable comprising conductors which are arranged around a central dummy conductor. An inner jacket and an outer jacket are provided. Between the actual conductors a so-called separation layer is provided which is made of wax or talc or mica. However, the inner jacket does not surround a conductor bundle but fills the spaces between the conductors. No grounding conductors are mentioned.
Furthermore, DE-A-3 326 986 shows a cable construction where conductors are surrounded by an insulation and further conductors as well as protection conductors and rubber enforcements are provided within a conductor coating which lies underneath an outer jacket. Interstices are filled with an oil graphite.
Accordingly, there is a need to provide a flexible power cable for use between motor control devices and the motors that they control, which minimizes electromagnetic noise and radio frequency interference and is capable of withstanding voltage or current spikes produced by the devices.
This object is achieved by providing a flexible power cable including a plurality of power conductors, each having insulation thereon and being arranged to form interstices between adjacent ones of the power conductors. Each of the power conductors includes a plurality of conductor strands. An insulated grounding conductor is provided in each interstice and together with the power conductors define a conductor bundle. An inner jacket surrounds the conductor bundle. A flexible, braided sheath member surrounds the inner jacket and is constructed and arranged to limit transmission and susceptibility to electromagnetic and radio frequency interference. An outer jacket surrounds the braided sheath member. The insulation of the power conductors and of the grounding conductors is lubricated so that the power conductors and the grounding conductors may move relative to each other and with respect to the inner jacket upon flexing of the cable.
Preferably, each of said power and/or grounding conductors can include a plurality of conductor strands.
Preferably, said outer jacket is a polymeric jacket.
Other objects, features and characteristic of the present invention, as well as the methods of operation and the functions of the related elements of the structure, the combination of parts and economics of manufacture will become more apparent upon consideration of the following detailed description and appended claims with reference to the accompanying drawing, all of which form a part of this specification.